libunwind 1.6.2-0 assumes 4k page sizes and crashes on systems with bigger page sizes

[Tobias Heider]

[ Impact ]

 * On kernels with page size > 4K Xorg (and presumably other applications
   relying on libunwind) crashes on startup. This affects anyone
   running the official arm64 generic-64k kernel or custom non 4k kernels
   (as used by e.g. apple silicon).

   The exact error I am seeing in the logs is:

Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE) 0: /usr/lib/xorg/Xorg (OsLookupColor+0x188) [0xaaab456ca998]
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE) unw_get_proc_info failed: no unwind info found [-10]
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE) Segmentation fault at address 0x0
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: Fatal server error:
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE) Caught signal 11 (Segmentation fault). Server aborting
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: Please consult the The X.Org Foundation support
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: at http://wiki.x.org
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: for help.
Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)

  I have not found a workaround other than using wayland (which has other
  limitations). To reproduce use a kernel configured with a page size of
  16K (CONFIG_ARM64_16K_PAGES=y or CONFIG_ARM64_64K_PAGES=y or) and try
  to start "Ubuntu on Xorg" in gdm.

[ Test Plan ]

 * Make sure Xorg doesn't crash on 4K, 16K and 64K kernels.

[ Where problems could occur ]

 * We will have to make sure the fixed version still works with 4k
   kernels. The patch is already widely in use so the risk seems low if
   we test properly.

[ Other Info ]

 * The lunar version ships the bug fix synced from debian

 * Debian bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1026217

 * Upstream fix: https://github.com/libunwind/libunwind/commit/e85b65cec757ef589f28957d0c6c21c498a03bdf

[Tobias Heider (tobhe)]

Attached is a debdiff with the backported fix from lunar

[Simon Quigley (tsimonq2)]

Hi Tobias, thanks for your interest in this bug!

While I did not do a code review of the patch itself, I have some general feedback on the diff and the bug report itself that I hope you find helpful:
 - The version should be 1.6.2-0ubuntu1.1 instead of 1.6.2-0ubuntu2, following Stable Release Update guidelines. Generally speaking, 1.6.2-0ubuntu2 would only be appropriate for the development release.
 - As a sponsor, I prefer DEP-3 headers[1] over Git patch headers, so the patch stays machine-readable (and so bug references such as this one and the Debian bug can be inserted).
 - The bug report description should be more thorough; does Xorg hard-crash, or is there a workaround? Could you provide specific instructions to reproduce the failure, please? Same with verification steps.

For now, I'm unsubscribing the sponsors team. Please resubscribe the team once most (or all) of these points have been addressed.

Thank you!

[1] https://dep-team.pages.debian.net/deps/dep3/

[Tobias Heider (tobhe)]

New diff as requested by @tsimonq2. I changed the version to 1.6.2-0ubuntu1.1 and added a DEP-3 header instead of taking the unmodified patch from lunar as I did previously.

[Simon Quigley (tsimonq2)]

LGTM, sponsored, thank you!

[Timo Aaltonen (tjaalton)]

Hello Tobias, or anyone else affected,

Accepted libunwind into kinetic-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/libunwind/1.6.2-0ubuntu1.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-kinetic to verification-done-kinetic. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-kinetic. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

[Tobias Heider (tobhe)]

I have installed libunwind=1.6.2-0ubuntu1.1 on my 16K page arm machine and verified that the xorg crash is indeed fixed with this version. To make sure it also does not negatively affect other hardware configurations I also tested on my amd64 desktop and found that everything still works without regressions after the update.

To test the functionality of the library I installed libunwind-dev=1.6.2-0ubuntu1.1 and ran the libunwind examples from https://github.com/daniel-thompson/libunwind-examples
The results look good:

$USER:~/libunwind-examples$ ./unwind-local
0x55d35d0d644c: (cmp+0xe)
0x7f927ac3d33c: (bsearch+0x5c)
0x55d35d0d61fc: (main+0x5c)
0x7f927ac23510: (__libc_init_first+0x90)
0x7f927ac235c9: (__libc_start_main+0x89)
0x55d35d0d6245: (_start+0x25)

[Ubuntu SRU Bot (ubuntu-sru-bot)]

All autopkgtests for the newly accepted libunwind (1.6.2-0ubuntu1.1) for kinetic have finished running.
The following regressions have been reported in tests triggered by the package:

linux-oem-5.17/5.17.0-1003.3 (amd64)
linux-lowlatency/5.19.0-1018.19 (amd64, arm64)
linux-ibm/5.19.0-1017.19 (amd64)

Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-migration/kinetic/update_excuses.html#libunwind

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

[Tobias Heider (tobhe)]

The detected regression looks like a false-positive caused by another dependency. The reported error is `/bin/sh: 1: python: not found` which does not seem to be caused by the changes in libunwind.

[Launchpad Janitor (janitor)]

This bug was fixed in the package libunwind - 1.6.2-0ubuntu1.1

---------------
libunwind (1.6.2-0ubuntu1.1) kinetic; urgency=medium

  * Backport debian bug fix for dynamic page size (LP: #2004039)
    - Add upstream change to determine the page size dynamically,
      thanks to Thomas Glanzmann. (Closes: #1026217)

 -- Tobias Heider <email address hidden> Mon, 30 Jan 2023 11:30:11 +0100

[Brian Murray (brian-murray)]

The verification of the Stable Release Update for libunwind has completed successfully and the package is now being released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.